Laser beam welding is a technique for irradiating a workpiece (for example, two steel plates) with a laser beam to locally melt and solidify the workpiece, thereby joining the workpiece. Laser welding inspection is an inspection for determining whether a welding defect exists in a welded portion of the workpiece which is formed by the laser beam welding.
There is publicly known a laser welding inspection which includes irradiating the welded portion of the workpiece with a laser beam for inspection, receiving a return light of the laser beam for inspection from the welded portion, and determining, based on intensity of the return light, whether the welding defect exists in the welded portion. For example, JP-A 2012-213806 discloses a laser welding inspection for determining whether the welding defect exists in the welded portion by a time-periodical change in intensity of the return light.
In the laser beam welding, there is generated a phenomenon in which, when the welded portion is irradiated with the laser beam for inspection, metallic vapor is generated near a laser irradiation point, and the generated metallic vapor flutters in a smoking manner from the welded portion.
The return light is also generated from the metallic vapor.
Therefore, in the conventional laser welding inspection disclosed in JP-A 2012-213806, when the return light generated from the metallic vapor is detected for a large region (monitor diameter) where the return light is recognized in the welded portion relative to the laser focal diameter, the detected return light from the metallic vapor has an influence on the time-periodical change, which results in a risk of falsely determining whether the welding defect exists in the welded portion.
Thus, in the conventional laser welding inspection, sometimes the false determination of the welding defect is made. Therefore, an unmanned laser welding inspection step has not been performed yet. In the laser welding inspection, there is a demand for improving accuracy for determining the welding defect without detecting the return light generated from the metallic vapor.